Various types of such structures are known. For instance ducts which are to be maintained cool despite high external temperatures are described in EP-A-544, 419 and comprise inner and outer skins with thermal insulation material between them. In particular, the thermal insulation material is a microporous material and the skins are of stainless steel or galvanised steel.

In DE-A-4, 218,464 a constructional element is a sandwich of two steel plates with a heat-resistance fibrous or other insulating material which is fused to an enamel inner face on each of the plates.

In DE-A-4,137, 241 a multilayer sandwich is formed between two outer layers of steel or aluminium each coated internally with sound-absorbing or fire-resisting material, and provided with a mineral wool or foamed filling in between.

In W089/01864 a fire-resistant tunnel is provided with an inner wall of metal sheet and an outer closed cell polyethylene foam, the metal sheet preferably being of steel and bonded to the foam by an adhesive which may have a higher melting point than the foam.

In addition to these specialised constructions, there are many instances where potentially very hot gases are confined to a chosen area by a steel sheet (for instance in the form of a duct) and the surrounding area is protected from the heat in the confined area by a layer of mineral wool insulation on the outer surface of the steel sheet.

Examples are ventilation ducts, including ducts for conveying flue gases away from a furnace.

The conditions to which the steel sheet is exposed would cause rusting if untreated steel was used and so it has been conventional practice to use galvanised steel in such fire protection structures. A description of such a

structure using galvanised steel is published in DIN 4102 part 4.

The mineral wool insulation is secured to the steel sheet either by direct bonding to the galvanised steel or by being clamped around the duct or otherwise mechanically secured to the outer surface of the sheet.

The insulation in practice is usually of mineral wool because of the necessity to reconcile cost with the need to provide good insulation against very high temperatures.

For any particular structure, a fire resistance rating is determined by observing the temperature on the unexposed surface of the mineral wool insulation (the"cold side") when the other side of the steel sheet (the"hot side") is subjected to defined high temperature conditions.

It would naturally be desirable to be able to improve the fire resistance rating of the structure in a simple and economic manner, since this would allow either better fire protection or would allow the maintenance of equivalent fire protection but with less, or less efficient, mineral wool insulation.

According to the invention, we improve the fire resistance rating of a fire protection structure comprising mineral wool covering a steel plate by using a steel plate of aluminium-coated steel instead of the conventional galvanised steel. We surprisingly find that when the hot side is exposed to high temperatures the use of aluminium- coated steel in place of galvanised steel improves the insulation rating of the structure. In particular it defers the rise in temperature on the cold side by a significant time and/or it results in depressing the temperature on the cold side surface of the insulation for a significant time.

It is very surprising and valuable that changing from galvanised steel to aluminium-coated steel defers the inevitable rise in temperature on the cold side. It is very surprising and valuable that, even when the temperature does rise, the replacement of the galvanised

steel by aluminium-coated steel results in the external temperature being held for a significant time at a lower temperature.

The invention includes both the use of aluminium- coated steel for improving the fire resistance rating, as described above, and also fire protection structures which are constructed to benefit from this use.

Broadly, the invention includes any fire protection structure which comprises an aluminium-coated steel sheet having a layer of mineral wool insulation on one surface.

The steel sheet is intended to confine the area where fire or other high temperature conditions may exist.

The structure may be, for instance, a fire wall or a fire door, wherein the wall or door is formed with the aluminium-coated steel sheet on the surface of the wall or door which is likely to be exposed to fire and the layer of mineral wool insulation on the reverse surface of the steel sheet, distant from the likely exposure to fire.

The invention is of particular value when the ultimate fire protection structure is a duct. The duct (which may also be described as a shaft) may be a service duct, e. g., for electric cables, but preferably it is a ventilation duct. This may be a duct for the escape of flue gases from a furnace or other combustion apparatus. Accordingly the structure may be an insulated ventilation duct (or shaft) comprising an inner wall formed of the aluminium-coated steel, and provided with the mineral wool insulation secured to the steel.

This insulated duct may be a structure which has been made by constructing a duct of aluminium-coated steel and then cladding this steel externally with the mineral wool insulation. Accordingly, the duct may be made in conventional manner except that aluminium-coated steel is used instead of galvanised steel.

The steel sheet may be from 0.33 to 5. Omm thick, preferably from 0.4 to 1. 3mm thick. The aluminium coating maybe from 15 to 60 micrometer thick. Suitable grades of

aluminium coated steel include aluminium silicium alloys and aluminium alone.

It is naturally desirable that all internal steel surfaces of the duct should be formed of aluminium-coated steel but, if desired, some of the surfaces can be formed of galvanised steel, or other forms of steel or structural metal, provided that the amount of the surfaces which are not of aluminium-coated steel is not sufficiently high to cause a significant deterioration in the fire protection properties of the structure.

The cladding of the steel duct with the mineral insulation can be conducted in conventional manner, for instance by bonding or mechanically fixing appropriately shaped mineral wool batts to the outer surfaces of the steel duct. Fixing may be by, for instancde, stud welted pins and washers, or by straps on the external surface.

The methods are broadly applicable, for instance when the structures are to be fire doors or walls.

The invention also includes fire protection structures which are suitable for constructing a ventilation duct and which each comprise an aluminium-coated steel sheet with mineral wool insulation secured to one surface, which would become the outer surface when the duct is constructed.

These elements may take various forms. For instance they may be plate-like elements which may be flat or curved and which are suitable for constructing a side wall of a ventilation duct. They may be ring shaped elements of any open cross-section (for instance circular or rectangular) suitable for constructing the entire peripheral wall of a ventilation duct as a result of stacking an appropriate number of the ring-shaped elements one on top of the other.

These ring-shaped elements may, for instance, have an axial length up to 2 metres. Alternatively, the elements may have a greater axial length than this and can be considered to be lengths of duct suitable for fitting with other lengths in order to form the ventilation duct.

With all these structures which are to be assembled to form the duct, the insulation secured to the steel sheet may be secured by mechanical fastening arrangements or by adhesion or in any other convenient manner.

One way of forming a ventilation duct from ring-shaped elements is described in, for instance, W093/11327.

Accordingly the duct can be made from elements broadly as illustrated and described therein except that the inner walls of the ring-shaped elements would be formed of aluminium-coated steel, and the outer walls need not include the outer metal sheets.

Although it may not be necessary from the point of view of fire protection, the surface of the mineral wool insulation distant from the steel sheet (the"cold"side) may itself be covered with a protective layer, which may be of plastic or metal foil or may be of a metal sheet. This covering may be selected according to the intended use of the structure, for instance as a fire wall, fire door or duct.

Preferred structures are sandwich panels which comprise a sandwich of the mineral wool insulation between the aluminium-coated steel sheet and another metal sheet.

This sheet may also be of steel, for instance galvanised steel or aluminium-coated steel.

There may be a facing material (such as painted or non-painted aluminium foil or glass fiber tissue or mesh) on the"hot"side of the steel sheet but this is generally undesirable. There may be a functional layer between the sheet and the insulation, but it is usually unnecessary (except as a possible bonding layer).

The mineral wool insulation may be of glass fibres but is preferably of rock fibres. The insulation is typically from 2 to 15cm thick and typically has a density of from 30 to 350kg/m3. Preferred mineral wool insulation is rock wool.

In another embodiment of the invention an endothermic material (e. g. , magnesium hydroxide, calcium carbonate),

which liberates carbon dioxide and/or water at temperatures above 200°C is included in the mineral wool. A mineral wool product of this type is described in W099/51536.

As an example of the benefits of the invention, the fire protection properties of two structures were determined. One structure was 0. 7mm zinc-coated steel plate secured to a 100mm mineral wool slab having a density of 165kg/m3. The other structure was identical except that the 0.7mm zinc-coated steel plate was replaced by a 0.7mm aluminium-coated steel plate. The two structures were individually subjected to a conventional fire-rating test with the metal surface exposed in a furnace to a time- temperature curve as in ISO 834, and the temperature of the outer surface of the mineral batt was determined at different times.

The change in temperature with time is shown in the graph which is the accompanying drawing.

It will be noted that after the rise in temperature, which is slightly delayed, the temperature for the next hour after that was about 10°C less, all as a result of changing merely from conventional galvanised steel to aluminium-coated steel.